Hello,
I have created OAuth 2.0 authentication to generate Access Token. I see the Access Token is saved in a json file. How the Json file can be saved locally or from where I could retrieve the Json file ? Please find the screenshot.
Welcome to the Postman Community! 
Not sure what you mean by saved in a JSON file? Do you mean the exported Collection?
I would strongly recommend not hardcoded any sensitive data (access tokens, client id and secrets) in your collection. You should use the Current Value of a scoped variable or better yet, use the Postman Vault to store all these.